The invention relates to the regeneration of ion exchange materials.
High purity water can be obtained from mixed-bed water treatment units such as condensate polishers in boiler water treatment plant. In order to maintain the quality of the treated water, it is necessary to minimise the deleterious effects which occur because either type of ion exchange material is unavoidably contacted by the regenerant appropriate for the other type. The ion exchange materials are classified before regeneration and cross-contamination of either type of material by the other to some degree is unavoidable. For example, contaminant cation type material present in the anion type is converted to the sodium form by the sodium hydroxide regenerant used to regenerate the anion type material and may give rise to a leakage of sodium ions into the treated water to the detriment of the boiler and the turbine. Sodium ions are quite readily displaced from the sodium form cation material by other ions.
Prior proposals to avoid such effects have involved regenerating the materials in separate vessels. Such methods involved transferring the upper anion material layer from a separator vessel, in which the materials had been classified, to an anion regeneration vessel. The transfer of anion material generally results in the transfer of relatively large amounts of contaminant cation material to the anion regeneration vessel. Typically, contaminant cation material could be 5% by volume of the material transferred. In some methods the regenerated anion material was treated, for example, with ammonia or calcium hydroxide to displace the sodium ions from the contaminant cation material. In another method the anion material was regenerated using a regenerant having a density intermediate the densities of the two types of material, giving a separating effect which removed the contaminant cation type material.
In an alternative proposal, described in U.S. Pat. No. 4,298,696 issued to Emmett, such cross-contamination of materials is minimised. U.S. Pat. No. 4,298,696 describes a method in which, following classification, material is transferred from the bottom of a separator vessel and the transfer flow is monitored to determine when one type of material has left the separator vessel. Transfer of materials by this method leaves the interfacial region between the materials relatively undisturbed so that contamination of one type of material with another is minimised. The interfacial region can comprise an inert material having a density intermediate the densities of the cation and anion materials. Alternatively, the interfacial region can comprise anion and cation materials in which case the interfacial region is isolated from the relatively pure volumes of anion and cation materials at least during regeneration of those materials.
However, even using the improved method described in U.S. Pat. No. 4,298,696, it has been found that a relatively small amount, typically of the order of 0.2 to 0.5% by volume, of cation material may be dispersed in the anion material. To ensure that customer requirements on water purity are met, it is preferable to minimise any contribution even that small amount of contaminant cation material may make to sodium leakage. It is particularly important to minimise such contribution when ammonia is added to boiler condensate to raise the pH of the condensate to minimise corrosive effects. In that situation, as the ammonia exhausts the cation material, ammonium ions progressively displace sodium ions from the cation material down the bed until the sodium ions leak into the treated water. However, the cation material still functions as an exchange material with respect to sodium and other ions even though it is in the ammonium form when, for example, a condenser tube leaks and introduces those ions into the top of the bed. Operation of the condensate polisher in such circumstances is usually referred to as operation through into the ammonia cycle.